Many types of non-aqueous rechargeable lithium batteries are used commercially for consumer electronics applications. Typically, these batteries employ a lithium insertion compound as the active cathode material, a lithium compound of some sort (eg. pure lithium metal, lithium alloy, or the like) as the active anode material, and a non-aqueous electrolyte. An insertion compound is a material that can act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms).
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Currently available lithium ion batteries are high voltage systems based on LiCoO.sub.2 cathode and coke or graphite anode electrochemistries. However, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material. These batteries employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
The excellent reversibility of this insertion combination makes it possible for lithium ion batteries to achieve hundreds of battery cycles. However, a gradual loss of lithium and/or buildup of impedance can still occur upon extended cycling for various reasons. This in turn typically results in a gradual loss in delivered capacity with increasing cycle number. Researchers in the art have devoted substantial effort to reducing this loss in capacity. For instance, co-pending Canadian patent application serial number 2,150,877, filed Jun. 2, 1995, and titled "Use of P.sub.2 O.sub.5 in Non-aqueous Rechargeable Lithium Batteries" discloses a means for reducing this loss which involves exposing the electrolyte to P.sub.2 O.sub.5. However, P.sub.2 O.sub.5 shows at best only limited solubility in typical non-aqueous electrolytes and can be somewhat awkward to use in practice. Alternatives which are soluble may be more convenient, but it is unclear why such P.sub.2 O.sub.5 exposure is effective and hence what compounds might serve as effective alternatives.
B.sub.2 O.sub.3 is a common chemical that is extensively used in the glass industry, and its properties are well known. B.sub.2 O.sub.3 has also been used in the lithium battery industry for a variety of reasons. In most cases, the B.sub.2 O.sub.3 is used as a precursor or reactant to prepare some other battery component. However, Japanese published patent application 07-142055 discloses that lithium batteries can show improved stability to high temperature storage when using lithium transition metal oxide cathodes which contain B.sub.2 O.sub.3. Also, co-pending Canadian patent application serial number 2,175,755, filed May 3, 1996, and titled "Use of B.sub.2 O.sub.3 additive in Non-aqueous Rechargeable Lithium Batteries" discloses that B.sub.2 O.sub.3 additives can be used to reduce the rate of capacity loss with cycling in rechargeable lithium batteries and that this advantage can be obtained by having the additive dissolved in the electrolyte. However, the reason that the B.sub.2 O.sub.3 additive resulted in an improvement with cycling was not understood.
B.sub.2 O.sub.3 commonly exists in a vitreous or glassy state. The structure is complex and is believed to consist of sheets of randomly oriented, 6 membered (BO).sub.3 boroxine rings which are connected by additional bridging oxygen atoms. (Crystalline B.sub.2 O.sub.3 can be obtained, but only with significant difficulty. Crystalline B.sub.2 O.sub.3 also has a complicated structure consisting of linked sets of zig-zag chains which form a three dimensional network structure.)
Certain other compounds containing boron, oxygen, carbon, and hydrogen (eg. trimethoxyboroxine, trimethylboroxin, trimethyl borate, tri-tert-butyl borate) have been used in the preparation of other compounds, particularly polymers. For instance, trimethoxyboroxine has been used to promote cross linking of silanes for Si-Si bond formation (PCT International Patent Application Serial No. WO9615080), as a catalyst for producing olefin polymers (European Patent Application EP705848), and to improve the melt stability of high molecular weight polycarbonates (Japanese laid-open patent application JP 06263866).
In battery and/or fuel cell applications, compounds containing boron, oxygen, carbon, and hydrogen such as trimethyl borate have been used as a precursor in a process to make an electrode substrate. For instance, in Japanese laid-open patent application JP 07105955, a precursor B-containing compound was kneaded in with the other electrode components before heat treating the mixture to 1000 degrees C. Boron-oxygen-carbon-hydrogen containing compounds have also been used in the preparation of lithium haloboracite (lithium-boron-oxygen-halogen containing material) solid electrolyte films for battery usage. However, it appears that these compounds have not heretofore been used directly in lithium batteries as additives or for any other purpose.